The present invention relates generally to the field of ceramics, and more specifically to a cementitious composition containing a dispersed seeded phase and a method and apparatus for producing seed crystals.
Concrete is one of the most ubiquitous of all structural materials, consisting generally of aggregates, such as rock or gravel, bound together in a cement matrix. While the aggregate phase comprises about 80% of the volume of the concrete, it is the cement binder phase that is most important regarding the physical properties and ultimate performance of the concrete.
There is a wide variety of different cements, including organic polymer cements, amorphous cements, and ceramic cement compositions. Ceramic cements are generally mixtures of water and reactive metal oxides that undergo chemical reactions causing them to harden and fasten after they are mixed and allowed to set. In addition to providing the binding matrix for concrete, cements have a variety of familiar uses, such as glues or adhesives for bonding porous materials, providing the bonding layer that holds bricks together to form walls, and as structural building materials such as patio or garage slabs. The cement of choice for the majority of commercial uses is Portland cement, a mixture of water and calcined lime (CaO) and silica (SiO2)-containing minerals. Upon curing, the primary constituents of Portland cement are dicalcium silicate (2CaOxc2x7SiO2), tricalcium silicate (3CaOxc2x7SiO2), and tricalcium aluminate (3CaOxc2x7Al2O3) phases and a ferrite phase containing calcia (CaO) and alumina (Al2O3). In commercial Portland cement, none of these phases are chemically pure; rather, they are solid solutions with such impurities as Mg and Al dispersed throughout.
Portland cement has the commercial advantage of being relatively cheap to produce and easy to mix and pour. Part of the reason Portland cement is so cheap is because the silica-containing mineral component may come from a wide variety of sources, usually silica-containing clays, and also because these clays are not required to be especially pure or consistent.
Portland cement also suffers from some disadvantages, with inconsistency of the physical properties of the cement being chief among them. The inconsistencies arise from the inherent inconsistency of the source materials, both in composition and quality. Typically, the raw constituents of Portland cement are ground clinkers containing hydraulic calcium silicates of varying compositions, calcium sulfates, and also various aluminates, manganates, and other impurities present in varying and uncontrolled amounts. Moreover, there is no real control of the grinding process, yielding cement powders with extremely variable particle size distributions (PSDs). The lack of consistency of the compositions and PSDs of the raw materials relates directly to a lack of consistency in the physical properties of the resultant Portland cement.
Portland cement also has the disadvantage of having a relatively high viscosity. While it is well adapted to pouring and spreading, Portland cement is too thick for most pumping and/or spraying operations. Another disadvantage of Portland cement is that it does not readily bond to itself. Portland cement-containing structures, such as cement driveways or road segments, must be formed in essentially one step. If there is an interruption in the forming of a Portland cement body sufficient to allow the cement to begin to cure, a structural discontinuity or xe2x80x9ccold jointxe2x80x9d can result. Moreover, Portland cement cannot be used to patch a Portland cement structure absent costly and time consuming surface pre-treatment at the patch interface.
Portland cement also has the limitation of having a slow setting or xe2x80x9cdryingxe2x80x9d time, during which the cement remains plastic and may be easily deformed. While the cement sets up and hardens, it is subject to damage and deformation by vandals, animals, and the elements; moreover, until the cement hardens it cannot support a load and in fact must itself be supported. This results in a potentially costly xe2x80x9csit and waitxe2x80x9d period during which further construction depending on the structural strength of the freshly poured cement form is necessarily suspended.
Another important limitation of Portland cement is that it is relatively soft, and is therefore not suited for those applications requiring a very hard surface, body, or bond. Portland cement also has limited toughness, relatively low tensile and fracture strength, and is relatively quickly worn down. Portland cement is also fairly porous and permeable to liquids, and thus quickly suffers from the deleterious effects of water intrusion and chemical degradation, such as from entrapped water expanding upon freezing and from seasonal exposure to de-icing salts.
There are other ceramic cements available that are tougher, harder, stronger, less porous, and/or chemically more stabile. For example, phosphate cements, resin-modified cements, and carbon-fiber composite cements are all harder and tougher than Portland cement. These cements also have the advantages of having more consistent and reliable physical properties. However, these cements have the disadvantages of being much more expensive and in much shorter supply than Portland cement. Hence, there is a need for a method of controlling the consistency of the physical properties of Portland cement and for altering the physical properties as desired. The present invention is directed at satisfying this need.
One form of the present invention contemplates controlling the physical properties of cement by adding a predetermined amount of a second phase to the cementitious precursor. The second phase is added in the form of seed crystals having controlled sizes, shapes, and compositions. Upon curing, the seed crystals provide growth sites for a second phase in the cement body. Curing is accelerated by the presence of the seed crystals. Control of the physical properties of the resultant cement is achieved by controlling its microstructure through the morphology, orientation, distribution and growth of the seed crystals.
One object of the present invention is to provide an improved method of producing a cement.
Another object of the present invention is to provide an improved method of producing small spherical particles via precipitating them in a drop-tube.
Still another object of the present invention is to provide an improved method of producing fine catalyst particles having high surface-area-to-volume ratios.
Yet another object of the present invention is to provide an improved method of coating particles and of making compounds and/or composites from precursors. Related objects and advantages of the present invention will be apparent from the following description.